Constant on-time pulse width modulation (PWM) is a standard control topology in power supply design. Constant on-time control configurations are often used for low duty cycle “Buck” converters (voltage step-down) because they provide fast transient response and do not require compensation for control loop stabilization. Constant on-time control is similar to so-called “hysteretic” control in that when the regulated output voltage falls below a reference threshold, a constant-on time DC/DC converter delivers energy to its output load. Unlike a hysteretic DC/DC converter where the amount of energy delivered to the load is set by a second reference voltage, the amount of energy delivered by a constant on-time DC/DC converter is determined by the on-time pulse of the DC/DC converter.
A standard or conventional constant-on control scheme exhibits significant PWM frequency variation, with variation in duty cycle similar to that exhibited in hysteretic control. So-called “pseudo constant on-time” control is a known technique for counteracting such variation. With pseudo constant on-time control, the PWM on-time is set as a function of the input and output voltages, such that the on-time pulse duration is fixed for given input/output voltages. The pseudo constant on-time technique is well known and does a reasonably good job at first order frequency compensation.
One drawback generally attending constant on-time type control is that it requires the feedback signal to the PWM comparator to be in phase with the converter's switching signals. Such synchronization typically is achieved by setting a minimum equivalent series resistance (ESR) value in the converter's output filter capacitor. Thus, operating with low value ESR capacitors generally requires additional external filter components, which is undesirable in terms of cost, space, and complexity.